(a) Field of the Invention
The present invention is related to a station-signal frequency indicating system for a radio receiver.
(b) Description of the Prior Art
Some modern radio receivers incorporate a station-signal frequency indicating system which electrically detects and displays the frequency of a station signal received.
Such known station-signal frequency indicating system usually has been designed so that a signal generated at a local oscillator in the receiver is repetitively counted by a counter to detect the local oscillation frequency. The counter is preset at a fixed numerical value corresponding to the intermediate frequency employed in the receiver. The count in the counter at the end of each counting period is displayed on a digital indicator as an indication of the station-signal frequency received. In practice, a correct indication is observed on the indicator, provided that the receiver is tuned exactly to the frequency of the station signal. However, in case the receiver is not exactly tuned to a station signal, the above system is no longer capable of indicating the exact station-signal frequency, and the system displays the tuning frequency itself of the receiver despite the fact that the receiver continuously receives the same station signal of a frequency slightly different from the tuning frequency. This is because the prior art system is essentially designed to always display an indication corresponding to the local oscillating frequency plus or minus the fixed preset number (numerical value) (intermediate frequency), irrespective of whether or not the receiver is exactly tuned-in for a given station signal. With the prior art system, furthermore, there has often arisen the problem that, because of the continuous running of the counter, noise signals that might be produced by the running counter would affect the receiver circuitry, interfering with the normal operation of the receiver.